Various methods have been proposed for preparation of sweepstakes contest awards, lottery tickets, promotional game cards, premium cards, and the like, containing hidden entries, such as numerals, messages, symbols, or the like, which can be revealed to the recipient by various means.
For example, U.S. Pat. No. 4,726,608 to Walton discloses use of an opaque coating over hidden indicia. The image of the indicia is later made visible by scratching off the opaque coating or by applying a solvent to disperse the coating. This system has the disadvantage of requiring extraneous solvents.
Copending U.S. application Ser. No. 07/987,694 entitled "Hidden Entry System and Use Thereof" to John C. H. Chang and Peter A. Walter filed Dec. 9, 1992 discloses a heat sensitive, autogenous chromogenic hidden system on sweepstakes documents or the like, and requires application of heat to develop the latent image, such as by use of friction resulting by scratching the latent image with the human fingernail. Since the latent image is heat sensitive, it can prematurely develop when the document is utilized with a laser printer to provide addressee information or the like on the sweepstakes document.